1. Field of the Invention
The invention generally relates to a shift control apparatus of an automatic transmission of a motor vehicle, and more particularly to such a shift control apparatus that effects coast downshift of the transmission while the vehicle is kept in a minimal driving state in which a weak engine brake is applied.
2. Description of Related Art
One type of a downshift control apparatus of an automatic transmission of a motor vehicle has been proposed which keeps the vehicle in a weak engine brake state when the automatic transmission is shifted down while the vehicle is coasting or decelerating with the accelerator pedal being released or not operated. An example of such a downshift control apparatus is disclosed in Japanese laid-open Patent Publication No. 11-287317. With this apparatus, certain engine brake force is kept constantly applied to the vehicle during clutch-to-clutch downshifts, whereby the downshifts to low-speed gear ratios do not cause excessive engine brakes or shift shocks.
When the automatic transmission goes through a clutch-to-clutch downshift, a release-side friction element (or offgoing friction element) is released and an engagement-side friction element (or oncoming friction element) is engaged around at the same time, and therefore the transmission tends to suffer from racing (a rapidly increase in the speed) of its input shaft or a temporary drop (or tie-up) of its output torque. In view of these situations, expensive control equipment that operates with considerably high accuracy may be required to control the engaging pressure for the release-side friction element when it is released and the engaging pressure for the engagement-side friction element when it is engaged. With generally available control apparatus for use in standard vehicles, however, the engaging pressures for the friction elements cannot be controlled with sufficiently high robustness with respect to disturbances, such as braking of the vehicle, and shift shocks may not be sufficiently reduced or suppressed.